Hydrogenation catalysts which enhance reactions between hydrogen and other compounds are a topic of significant interest. In particular, catalysts for CO/CO2 hydrogenations into higher oxygenates (C2+, i.e. ethanol, etc.) are of specific interest. These higher oxygenates are widely used as solvents, intermediates, fuel additives, and neat fuels, and producing these products selectively requires catalysts with specific properties. Typically, major byproducts from CO/CO2 hydrogenations are single carbon compounds, so a major challenge is the development of catalysts with higher selectivity towards the higher alcohols and oxygenates. There is particular emphasis on high selectivity catalysts for CO/CO2 hydrogenations acting in environments of CO, CO2, and H2, such as syngas, which require relatively high stability in the presence of a reducing environment. Generally a variety of catalysts, particularly the Group 6 through 11 metals, have been employed in CO/CO2 hydrogenations, but in many cases these catalysts generate broad, complex mixtures of hydrocarbons, oxygenated hydrocarbons, and carbon dioxide. Thus, there is a need for CO/CO2 hydrogenation catalysts that selectively generate higher alcohols/oxygenated hydrocarbons products.
Certain metals such as Rh have been extensively studied because of their generally high hydrogenation activities. For example, these catalysts have been shown to be the most active and selective for higher alcohol synthesis compared to alternatives based on modified copper, cobalt-molybdenum, or promoted Fischer-Tropsch catalysts. The activity and selectivity of the active metal catalysts can be increased by various factors, such as the presence of promoters, the choice of support, the synthesis method, the use of specific precursors, and other factors. Generally, optimum higher alcohols/oxygenated hydrocarbons formation requires a balance among the rates of CO dissociation, hydrogenation, and CO insertion. For example, promoters such as rare earth metals, alkali metals, and other transition metals play an important role in these elementary steps. Typically the promoters activate the oxygen atom of an absorbed CO molecule and weaken the C—O bond, leading to CO dissociation followed by a hydrogenation step to form CHx species. The mechanism for C—C bond formation leading to higher alcohols/oxygenated hydrocarbons also requires the atomic proximity of an activated, associatively adsorbed CO that can react with the CHx species. Subsequent hydrogenation of this initial C2 intermediate leads to higher alcohols or oxygenated hydrocarbons synthesis. The two sites where one forms CHx and the other an activated CO are catalytically distinct, but need to be atomically adjacent. As a result, the hydrogenation of CO to produce C2+ oxygenates such as ethanol is thought to require the atomic proximity of catalytic sites that activate CO in two ways: (i) dissociative adsorption of CO to produce surface carbon that is hydrogenated to form a surface CHx species and (ii) associative adsorption of CO, which is activated by the catalyst and couples with the CHx species to form the critical C—C bond.
Catalytic metals have also been substituted into certain crystalline oxides such as perovskites and pyrochlores in an effort to promote selectivity in CO/CO2 hydrogenations. See e.g., U.S. Pat. No. 4,312,955 to Bartley; and see U.S. Pat. No. 4,126,580 to Lauder; and see U.S. Pat. No. 4,863,971 to Broussard et al.; and see Tien-Thao et al., “Effect of alkali additives over nanocrystalline Co—Cu-based perovskites as catalysts for higher-alcohol synthesis,” Journal of Catalysis 245 (2007); and see Tien-Thao et al., “Characterization and reactivity of nanoscale La(Co,Cu)O3 perovskite catalyst precursors for CO hydrogenation,” Journal of Solid State Chemistry 181 (2008), and see Bourzutschky et al., “Conversion of synthesis gas over LaMn1-xCuxO3+λ perovskite and related copper catalysts,” Journal of Catalysis 124 (1990). Additionally, CO/CO2 hydrogenation catalysts have involved catalytic metals such as Co, Cu, and Rh supported by various structures such as La2Zr2O7, LaFeO3, La2O3, TiO2, SiO2, and Al2O3. See Kieffer et al., “Hydrogenation of CO and CO2 toward methanol, alcohols and hydrocarbons on promoted copper-rare earth oxides catalysts,” Catalysis Today 36 (1997); and see Chu et al., “Conversion of syngas to C1-C6 alcohol mixtures on promoted CuLa2Zr2O7 catalysts,” Applied Catalysis A: General 121 (1995); and see Fujiwara et al., “Hydrogenation of carbon dioxide over copper-pyrochlore/zeolite composite catalysts,” Catalysis Today 29 (1996); and see Fang et al., “LaFeO3-supported nano Co—Cu catalysts for higher alcohol synthesis from syngas,” Applied Catalysis A: General 397 (2011); and see Chuang et al., “Mechanism of C2+ oxygenate synthesis on Rh catalysts,” Topics in Catalysis 32 (2005). The efforts are generally aimed toward adjustment of the CO dissociation and insertion abilities of the Co, Cu, or Rh through varying promoter and support compositions. Variations in selectivities are typically attributed to the specific properties of the support, the promoter, the morphology of the metal, and the impact of the support on the reducibility of the metal.
Recently, the presence of an atomically adjacent ionic and metallic species (M0-M+) has been reported to enhance the coupling between undissociated CO and CHx and the selective formation of ethanol via ketene (H2C═C═O) or acetyl (H3C—C═O) intermediates. The higher oxygenated hydrocarbon selectivity is postulated to occur via the formation of a “tilted” CO species in which both the carbon and oxygen atoms are bound to the surface. One way in which these types of sites needed to produce higher alcohols/oxygenated hydrocarbons can be prepared is to use particular crystalline oxides such as a perovskite, pyrochlore, fluorite, or brownmillerite with particular catalytic metal sites, where catalytic metals are also doped into the perovskite, pyrochlore, fluorite, or brownmillerite and will have the M0-M+ coordination. Additionally, such perovskite, pyrochlore, fluorite, and brownmillerite materials allow various metals to be isomorphically substituted into the oxide structures providing, for example, basic sites that act to activate adsorbed CO. Further oxygen conductivity of these materials may enhance the ionic and metallic (M0-M+) species coordination. This property has also been shown to reduce undesired carbon formation. The use of such crystal oxides with the doped catalytic metal sites also promote a high degree of thermal stability in environments which may be highly reducing. Further, atomically adjacent ionic and metallic species (M0-M+) can be achieved by depositing the catalytically active metal(s) (M0) on the surface of the doped mixed-metal oxides.
Provided here is a method of hydrogenation utilizing a reactant gas mixture comprising a carbon oxide and a hydrogen agent, and a hydrogenation catalyst comprising a mixed-metal oxide with a metal site supported by and/or incorporated into the lattice. In an embodiment, the metal site is a deposited metal and the mixed-metal oxide supports the metal site. The metal site comprises a transition metal, an alkali metal, an alkaline earth metal, or mixtures thereof, and the conducting oxide comprises a perovskite, a pyrochlore, a fluorite, a brownmillerite, or mixtures thereof, typically doped at an A-site or B-site of the conducting oxide crystal structure. Contact between the carbon oxide, hydrogen agent, and hydrogenation catalyst under appropriate conditions of temperature, pressure and gas flow rate generate a hydrogenation reaction and produce a hydrogenated product made up of carbon from the carbon oxide and some portion of the hydrogen agent. The carbon oxide may be CO, CO2, or mixtures thereof and the hydrogen agent may be H2. In a particular embodiment, the hydrogenated product comprises an alcohol, an olefin, an aldehyde, a ketone, an ester, an oxo-product, or mixtures thereof.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.